1. Field of The Invention
This invention relates to a wet developer which is adapted for development of electrostatic latent images such as in electrophotographic processes.
2. Description of The Prior Art
In the field of image formation techniques, systems wherein light is selectively irradiated in a uniformly charged photoconductor according to image signals and the resultant electrostatic latent image is developed are generally called an electrophotographic process. Broadly, the electrophotographic process can be classified into a dry developing process and a wet developing process.
Principally, the dry developing process comprises merely spraying and depositing a colorant powder on an electrostatic latent image and is thus advantageous in good handling properties and good shelf stability. However, as is experienced, for example, in a video printer wherein electronic still photographs are printed, it has now been accepted that the dry developing process is inferior to the wet developing process in order to meet a demand for high-quality images which has been increased in recent years.
The wet developing process is a system wherein a liquid developer dispersing a dye or pigment used as a colorant in an insulating medium is used. According to the wet developing process, it is possible to provide resolution and gradation comparable to those of a silver salt photograph. Especially, where pigments are used as a colorant, the resultant image has good weatherability. This type of system has been extensively developed in various fields.
The developers widely used in the wet developing process are wet developers which make use of an insulating medium which is a liquid at normal temperatures such as, for example, saturated hydrocarbon-based Isoper G (Esso). This type of wet developer is hereinafter referred to as liquid toner.
Moreover, we have already proposed in Japanese Patent Application No. 63-156846 a wet developer wherein colorant particles are dispersed in electrically insulating organic materials which are solid at normal temperature and are capable of liquefaction by heating, so that the problems involved in the liquid toner, e.g. a difficulty in handling, complication in maintenance, and poor shelf life, can be solved. This type of developer is hereinafter referred to as solid toner.
Since the toner is stored as a solid, it is easy to handle with a reduced change in composition. In addition, the toner is molten by means of an appropriate means on used, so that it is possible to effect the wet development like ordinary liquid developers.
In wet developers used in the afore-stated wet developing method, resin ingredients are added either in liquid toners or in solid toners so that they are adsorbed on colorant particles to take an electric charge supplying agent therein. Polymers conventionally used as the resin ingredient are predominantly copolymers of styrene and dodecyl acrylate for the purpose of increasing the solubility in non-polar solvents.
However, such copolymers have so small in number of sites, where metallic ions are taken, that their interaction with an electric charge supplying agent is weak although such interaction is essential for wet developers. In addition, since any polar functional group is not present at the side chain, satisfactory results are not obtained with respect to the adsorption to colorant particles and the fixation to photosensitive films. At present, this disadvantageously leads to unsatisfactory resolving power, gradation and fixation. Especially, with the solid toner, heating is necessary for the development, with a tendency toward the shortage of adsorption of the resin ingredient to the colorant particles. The above disadvantages becomes more pronounced.
With the conventionally employed copolymers, it has to be added in an amount of approximately two to five times the amount of colorant particles, with an attendant problem that when the development is repeated, the composition may be changed and the resin may be fixedly deposited on a developing device.
Studies have been made on the resin ingredient contained in the wet developer, for example, in Japanese Laid-open Patent Application No. 61-156262. However, these studies have been directed mainly to fixation, not to the interaction with electric charge supplying agents and colorant particles as set forth before.